Page 2 of 2

Re: Front ERF vs Internal ERF? Minimum size?

Posted: Tue Jun 05, 2018 9:49 pm
by marktownley
Maybe the smoke / vapour we are seeing is just pollen in the air showing tube currents?

Re: Front ERF vs Internal ERF? Minimum size?

Posted: Thu Jun 07, 2018 4:05 am
by TheSkyBurner
today i moved the b-ccd filter 40mm closer to the focus, the reflection is now elliptical shaped and being dumped across two of my extension tubes (threaded and baffled). There is no smoke coming off of this now and the heat is effectively dissipated across the entire array of extensions. It is now functional as a cylindrical heat sink.

The reflection is now completely away from optical line of sight and is not causing any additional interference or reflections on the objective. The polarizer is detached, but I may still install a smaller one to experiment with filter clocking.

Re: Front ERF vs Internal ERF? Minimum size?

Posted: Sat Jul 07, 2018 6:49 am
by Rusted
This is a bit late but I have a warning about sub-aperture, internal D-ERFs:

There is a fiercely hot, refocused and unfiltered, reflected Solar image well in front of my 6" f/8 [120/10] objective.
I'm using a 90mm internal Baader D-ERF with PST H-a filtration in an old CR150HD donor.

Fortunately, my head has usually blocked the sun's light when I happened to glance into the objective while pointing at the Sun.
But an unwary hand waved in front of the objective would probably be burnt since it can't eclipse enough incoming sunlight.

A long dewshield might be a very good idea where the public has even "accidental" access to such an instrument.
An amateur solar observer, with children wandering about, should be very aware of this very real danger.
A small child's head would still allow annular sunlight to fall on a larger objective with potentially tragic consequences.
Most of a lens's collecting area is in the outer zones.

A full aperture D-ERF filter, fixed securely in front of the objective, would minimize the risk down to "normal strength" reflected sunlight.
The reflected glare would usually be enough to cause a person, or child, to desist from their "Darwin Awards" curiosity.

However, a reflected and re-focused image, from an internal D-ERF, might not give the "accidental" viewer any chance at all to withdraw before damaging their sight permanently.
And yes, you can easily set light to a strip of wood held a few inches in front of the objective on my 6" modified refractor!
Fortunately my telescope is very inaccessible and large enough to need a stepladder to reach the objective.

Logic suggests that the larger the internal D-ERF the further forward of the objective the Sun's re-focused image will be thrown.
I wouldn't be leaving stepladders anywhere near such an instrument when leaving it unattended.! :movie